Photochromic compounds are compounds which can reversibly change color and/or degree of light transmission when they are exposed to a source of exciting electromagnetic radiation, e.g., visible light or ultraviolet light most commonly. The compounds return to their original state of color and/or light transmission when the exposure source is removed and/or the compound is exposed to counteracting light. Many forms of photochromic compounds are known. For example, U.S. Pat. Nos. 3,100,778; 3,501,410; 4,220,708; 4,882,438; 5,532,361; and 5,543,533 describe a variety of photochromic structures.
One class of photochromic compounds are pyran-based in that these compounds incorporate a pyran moiety. Color changes in such compounds result from configurational changes in the pyran moiety. In one form corresponding to the ground state, the pyran moiety is closed and forms a six membered ring. Pyran-based photochromic compounds in the ground state are most commonly colorless. In another form corresponding to the excited state, exposure to light causes the pyran ring to open up. Pyran-based photochromic compounds in the excited, open state exhibit a second state of color. For example, some of the previously known pyran-based photochromic compounds display yellow, yellow-orange, or blue colors when in the excited configuration.
The closed configuration is generally more stable than the open configuration. Accordingly, in the absence of an exciting source of radiation, pyran-based compounds in the open state tend to revert back to the closed configuration. To help stabilize the open configuration, the pyran moiety is typically linked to moieties having extended .pi.-electronic systems. These stabilizing moieties are linked to the pyran ring in such a way that at least one such stabilizing moiety is linked to one side of the open pyran ring, and at least one other is linked to the other side of the open pyran ring. Examples of such compounds are known and have been described in U.S. Pat. Nos. 3,100,778; 3,501,410; 5,532,361; and 5,543,533.
Stability of pyran-based photochromic compounds still remains a concern, however. Many of the pyran-based compounds fade from a colored state to a colorless state too quickly. In other materials, a lack of stability is evidenced by weak colors. There is a need, therefore, for pyran-based photochromic compounds having greater stability and that display more vibrant, pure colors.
Temperature dependence is another concern. Many pyran-based photochromic compounds display photochromic properties only at temperatures well above or below ambient conditions. It would be desirable if such compounds could display photochromism over a wide temperature range, including ambient temperatures.
Many of the previously known photochromic compounds also lack water solubility, preventing their use in aqueous systems. Still others are made by reaction schemes having low yields, affecting the efficiency and economics of manufacture.